LCD displays need drive voltage levels far above the usual logic high or low levels from a typical application environment. Level shifters are needed to transform the logic levels of the control signals into positive and/or negative drive signals of an appropriate level that depends on a particular LCD display and can reach several tens of volts. Each level shifter channel has low-impedance output stages that achieve fast rise and fall times when driving the capacitive loading typically present in LCD display applications.
Typical level shifters for TVs and monitors may have multiple separate channels, some of which support gate voltage shaping to improve picture quality by reducing image sticking. This is usually implemented by generating flicker clock signals to determine exactly when gate voltage shaping should begin. In LCD displays using Gate-in-Panel technology, one flicker clock is needed for each pair of input signals. Since each pair of input signals is 180° out of phase, one flicker clock can be used for both. For example, a four-phase display requires two flicker clock signals; a six-phase display needs three flicker clock signals and so on.
In practical applications it is not always possible to provide all the flicker clock signals a display requires. This can be because of limited capabilities of the timing controller that generates these signals, or because of the limited number of connections supported by the level shifter IC. Furthermore, the conventional approach described above does not easily allow a system designed for one type of display (e.g. four-phase) also to be used for another (e.g. six-phase). If a dedicated flicker clock signal is not available for each pair of input signals, an additional signal can be generated to synchronize a single flicker clock signal at the start of each picture frame, but at the expense of complexity.